1. Field of the Invention
The present invention relates to a method for preparing electrodeposited nano-twins copper layer and the nano-twins copper layer prepared by the same, more particularly, to a method for preparing a nano-twins copper layer having a plurality of [111] surfaces on its surface and the nano-twins copper layer thereof.
2. Description of Related Art
Mechanical strength of metallic material increases generally when the size of the crystal grain is reduced to a nanoscale level. Some nano-scale levels of thin metal films can even have particular mechanical properties such—Young's coefficient. As a result, twins metal having nanoscale crystalline properties will be suitable for applications of through silicon via, semiconductor chip interconnect, packaging substrate pin through hole, metal interconnect (for example, copper interconnect), or metal materials on substrate.
In terms of electrical performance of microelectronic devices, electromigration has become a critical reliability issue of Cu wires. From the disclosure of the past research, three methods have been discovered to increase the electromigration lifetime of the wires.
The first method is to fabricate the Cu grain structure with a [111] preferred orientation, so as to significantly enhance electro-migration resistance, and reduce the possibility of formation of voids caused by electro-migration.
The second method is to increase the grain size to decrease the area of grain boundary, and to further reduce the migration path of atoms. The third method is to add metals having nano-twins structure into the wires. The atoms move along the direction of the moving electrons and result in electro-migration damage. Moreover, the electromigration rate can be retarded at twin boundaries. In the spirit of this working principle, nano-twins can inhibit the formation of voids in the interconnect, and directly improve the lifetime of electrical devices. In other words, the higher the nano-twins density inside the interconnect is, the better anti electro-migration will be.
Generally, nano-twins copper metal layer is prepared through physical vapor phase deposition (PVD) or pulse electroplating. But the twins material prepared by these known technologies illustrated above can only obtain miniscaled and irregular nano-twins, and suffer high production cost. In addition, PVD cannot fill Cu in trenches or vias with a high-aspect ratio. Hence, these known methods are not widely applied in mass production of semiconductor and electronic products. In other words, these methods still cannot be applied in industrial mass production.
Several researchers disclosed the details of forming nano-twins copper metal layers. For example, O. Anderoglu et al. disclosed a way for preparing a nano-twins copper metal layer structure by physical vapor deposition (PVD). The thickness of a single crystal grain can reach only hundreds of nanometer, and only applicable for use in preparing seed layers. (O. Anderoglu, A. Misra, H. Wong, and X. Zhang, Thermal stability of sputtered Cu films with nanoscale growth twins, Journal of Applied Physics 103, 094322, 2008). In addition, because physical vapor phase deposition inherently cannot plate a concaved trough of a high aspect ratio well and deposition duration is long, neither structure nor its preparative method is applicable for use in copper interconnects, through silicon via, or under-bump metallization (UBM), etc.
On the other hand, Xi Zhang et al. disclosed another way for producing nano-twins copper film through copper sulfate solution, and pulse electrodeposition device. One drawback of the known technology comes from the fact that the size of produced crystal grains is too small, the twins copper growth orientation cannot be controlled, and the pulsed electrodeposition rate is low. Hence, its economic benefits downgrade. (Xi Zhang, K. N. Tu, Zhang Chen, Y. K. Tan, C. C. Wong, S. G. Mhaisalkar, X. M. Li, C. H. Tung, and C. K. Cheng. Pulse Electroplating of Copper Film: A Study of Process and Microstructure, Journal of Nanoscience and Nanotechnology, VOL 8, 2568-2574, 2008).
U.S. Pat. No. 6,670,639B1 discloses a copper interconnection, wherein 50% of the crystal grains of the copper or copper alloy inside the interconnect are arranged in a uniform [111] crystal orientation, and connects with crystal grains of other crystal orientation to form a bamboo structure, and to form double crystal lattice on the wire surface. The efficacy of the structure illustrated is to increase high reliability and to lower production cost. However, although the prior art can offer high reliability and low production cost, it cannot offer the anti electro-migration as present with nano-twins at the same time.
U.S. Pat. No. 7,736,448B2 discloses a preparative method for twins-copper interconnect using “pulsed electrodeposition”. The density of the twins layer prepared thereof is high, but the crystal grain size is merely 300 nm-1000 nm, which are random, orderless small size isometric crystal grains. Furthermore, the range of operation for the electric current density of the disclosed pulse electrodeposition in the said technology is limited to be within 4 mA/cm2-10 mA/cm2, and the film plating deposition rate is overly slow. Therefore, the materials' economic value is deprived by the shortcoming illustrated above.
In summary, there are generally two drawbacks with the prior arts: (1) the grain orientation therein is difficult to master. Only copper grains with random orientations can be fabricated, and its effect on improving product efficacy is limited when used in interconnect or contact points; (2) prior arts' deposition speed is low despite of use of pulse plating or physical vapor phase deposition. Its deposition duration is long, efficacy is low, and production cost is high. In other words, it cannot compete against other products with respect to large-scale production.
Accordingly, the microelectronics industry needs a nano-twins copper metal layer having a [111] preferred orientation, so as to come up with the most favorable anti electro-migration for wires. At the same time, there is a need for nano-twins copper metal materials having excellent mechanical property, and the preparative method is also fast, and low cost. Moreover, the preparative method is compatible with current semiconductor manufacturing, which is believed to be in line for replacing directly the applicability value of the traditional interconnect or contact materials.